At the end of the day you need at least one battery, and preferably two. I don't think there is anything implicit that states that the battery is included.
Years ago when buying one of my first trailers I was asked if I'd like to add a second battery as an option. I didn't know much, but the logic seemed sound. The price was a couple of bucks different from what I could have bought a battery for elsewhere but it was installed, included a battery box, and was wired and tested properly.
Ever since that initial experience I have always indicated during the purchase process that the price I was negotiating would include a second installed battery, the implication being that there is already at least one in the deal. On my current deal I didn't even need to bring it up as it was already listed on the build sheet when I went over my order.
Some of these dealerships are used to getting so beat up that they are getting creative trying to add some profit back in the deal. I don't like it myself, but having been in sales a good portion of my adult life I've watched consumers devalue the role of a salesman in a purchasing process they are doing everything they can to increase revenue. This has resulted in an unbundling of services and fees in order to try to show the cheapest price possible for the basic unit. I don't like it... but it's the way things are. I highly doubt any dealer is going to not write the deal over the cost of the battery, but if they can get you to buy one they probably will... sad but true.
Congratulations on your new purchase and I hope the process goes well for you.
2009 F350 CC LB 4WD
2017 Jayco 355MBQS